1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a computer program product for performing image processing to achieve uniform color reproduction by an image forming apparatus.
2. Description of the Related Art
For an image forming apparatus, such as a color printer, color copier, or color printing machine, that forms color images on an image recording medium by using digital data on the basic colors CMYK, and the like, image processing called color management is performed on input data in the image forming apparatus in order to achieve uniform color reproduction. For color management, the color management framework proposed by the International Color Consortium (ICC) and its data format, ICC profile, are widely known.
As is the case with ICC profiles, often in color management for printing system with high nonlinearity, the basic unit of conversion is made up of a combination of a multidimensional look up table (LUT) that associates an input color space with an output color space and a one-dimensional LUT that linearizes input and output thereof for respective channels of CMYK colors, and the like. The one-dimensional LUT for an input/output channel is called a TRC, which means tone reproduction curve or tone response correction. A TRC does not always need to be implemented as an LUT; however, in many cases, it is implemented as an LUT because of its simplicity of implementation and versatility.
The ICC framework provides, for the color conversion as described above, a method in which one color space is converted to another color space through a device-independent color space called a profile connection space (PCS) and a method (called a device link) of direct mapping not through a PCS using a TRC of an input/output channel and a multidimensional LUT. In the ICC, a color space of a CIE1976Lab color system or CIE1931XYZ color system is used as a PCS or its subspace is used. In the following descriptions, data that stores a TRC of an input/output channel and a multidimensional LUT used to perform color conversion between channels as described above, not only that of the ICC, is referred to as a color profile or simply as a profile. (According to the definition of the present specifications, the ICC profile format is a package of a plurality of profiles in accordance with rendering intent and conversion direction.)
In the color management framework as described above, an image forming apparatus, which supports, as image forming targets, a plurality of types of print sheets or a plurality of types of media (image recording media), needs a different color profile for each medium with different color reproduction characteristics. Furthermore, even for media with the same color reproduction, there is a need for profiles that are adjusted according to print conditions, such as the number of halftone dot lines, the type of content, such as a character, graphic, or photo, or compression intent of a color gamut; therefore, the required number of profiles tends to be large in response to the requirements of customers who wants to deal with a variety of media. In terms of cost and management, it is difficult for a maker who offers image forming apparatuses to provide profiles that adapt to all media and output conditions. Thus, a maker provides profiles corresponding to some typical media and some typical output conditions and leaves to customers the creation of profiles corresponding to other media and other conditions.
For the creation of profiles, although constructing a color reproduction model that is mapping from a device color space to a PCS is relatively easy, mapping in the opposite direction from a PCS to a device color space (a color space with CMYK parameters in most image forming apparatuses) is technically difficult because it includes an unstable process of corresponding a color outside the color gamut irreproducible by the device with a reproducible color inside the color gamut and therefore an unexpected defect can easily occur on the constructed mapping. Such a mapping defect is likely to occur in adjustment to reflect, in particular, user's preference on a reproduced color, and this is the factor that requires a certain amount of experience.
Conventional methods disclosed in, for example, Japanese Patent Application Laid-open No. 2008-153810, Japanese Patent No. 4086442, and Japanese Patent No. 4135599 are proposed to solve the problem of the increasing number of profiles corresponding to a variety of media as described above. Japanese Patent Application Laid-open No. 2008-153810 discloses the idea that a sheet of paper whose characteristics are close to those of a user' sheet of paper is selected from regular sheets of paper whose characteristics are already known and conversion data on image data is combined by using it. Japanese Patent No. 4086442 discloses a method of sheet adaptation in which linear transformation is performed on the input RGB side. Specifically, it discloses that a unit which performs RGB to RGB conversion in an input space so as to correct a difference depending on a medium is provided as a media converting unit and that an affine transform matrix is used as a correcting unit in the above case. Japanese Patent No. 4135599 discloses a method of reducing, by using a sheet difference correction curve, a reproduction color difference between images output from different image output apparatuses using sheets of paper of different white color.
The technology disclosed in Japanese Patent Application Laid-open No. 2008-153810 is a method to generate conversion data for device-link type RGB to CMYK conversion for a user's sheet of paper. The conversion data is generated by a combination of RBG to CMYK conversion for a known regular sheet of paper, CMYK to PCS conversion for the same regular sheet of paper, and PCS to CMYK conversion for a user's sheet of paper. Thus, it is essential for the combination to separately obtain a profile for a user's sheet of paper, especially PCS to CMYK conversion for which generation requires experience as described above; therefore, the user's difficulty for constructing profiles are not essentially resolved.
The method of sheet adaptation disclosed in Japanese Patent No. 4086442 is based on the assumption that the color reproduction characteristics between the sheets of paper for which conversion is performed are close enough to each other; therefore, there is a problem in that the method cannot be applied if there is a significant color gamut difference therebetween or if a nonlinear relation occurs therebetween such that they cannot be adequately approximated to each other by affine transformation.
In the method disclosed in Japanese Patent No. 4135599, first, a reproduced color in a PCS (a Lab space) of a first image output apparatus using a reference sheet of paper is made to correspond with a reproduction color value in a PCS of a second image output apparatus by cyclically undergoing PCS to CMYK to PCS and using a profile of the second image output apparatus for a target sheet of paper, and in order to reduce any error between the reproduction color value and a reproduction color value in the PCS of the first image output apparatus, tone value correction is performed on input CMYK values with respect to the reproduction color value of the first image output apparatus. In this case, mapping between the PCS and the CMYK device spaces as described above functions as mapping for color gamut compression that makes an arbitrary point in the PCS with a point within the color gamut of the second image output apparatus. Therefore, in this case, too, it is essentially necessary to prepare in advance a profile that includes PCS to CMYK conversion on the second image output apparatus side for the user's sheet of paper, and in addition, the sheet correction thus obtained causes the color gamut of the second image output apparatus to be limited to the range that is common with the color gamut of the first image output apparatus. This causes a problem in that the practical color gamut becomes small especially when a large variety of sheets of paper is to be handled.